It is common that wellbores through earthen formations, such as in the petroleum industry, are lined with strings of tubulars. Such a tubular string is termed herein casing but can sometimes be termed a liner, depending on the position of the string in the well. Both when running in casing and when drilling the borehole with casing, which is to remain downhole, it is sometimes useful to connect a bit to the lower end of the string to open up, drill or extend the borehole.
When a bit is used with a string of casing and the casing is intended to remain in position downhole, it is often desirable that the bit be retrievable through the casing string to surface. To permit the bit to be retrieved, the drill bit can have a maximum outer diameter less than the drift diameter through the casing. Such a bit can drill a borehole less than the outer diameter of the casing but requires a means for enlarging the borehole to permit passage of the casing. To enlarge the borehole diameter, sometimes the bit includes or is connected to one or more under reamer arms. In addition or alternately, cutters can be mounted on the lower end of the casing forming a section called a drill shoe. The drill shoe can be used with a pilot bit to make drilling runs.